Mechanisms of synaptic transmission in the caudate nucleus and hippocampus are being analyzed in developing and mature animals. Our working hypothesis is that synaptic mechanisms which become functional early in the development of the nervous system may be more easily in the absence of later maturing, competitive mechanisms. Acetylcholine responsiveness of caudate neurons in the adult have never been characterized. In the cat the dopaminergic nigrostriatal pathway is a slowly developing system. We have recorded excitatory responses from caudate neurons as early as the 6th postnatal day after microiontophoretic ejection of acetylcholine. In adult rabbits, the cellular site in the brain for the conversion of systemically administered L-DOPA to dopamine is being investigated. We are contrasting brain concentrations of monoamines after L-DOPA in animals with various brain lesions. In the hippocampus, neurotransmission through synapsis at the mossy fiber terminals on pyramidal cell dendrites is under study. We reported that these terminals are rich in zinc. The concentration of zinc increases rapidly between 18 and 22 days of age in the rat. The terminals were also rich in glutamate dehydrogenase and glutamic acid was released by electrical stimuli. Zinc chelators diminished the excitatory potential recorded on the hippocampal surface.